The present invention relates to a method of strengthening the neck portion of a hollow blow-molded bottle-shaped container of a biaxially oriented polyethylene terephthalate resin.
A hollow blow-molded bottle-shaped container of a biaxially oriented polyethylene terephthalate resin, abbreviated hereinafter to as "a PET", has excellent physical properties, and durability, and a wide range of applications. However these excellent properties such as superior physical properties, durability, etc. of the bottle-shaped container can be partly performed effectively only after the resin material is biaxially oriented.
For example, some of the physical properties and the durability of the neck portion of the bottle-shaped container which were blow-molded without almost being oriented at the time of molding were deteriorated compared with the body and the like of the bottle-shaped container which had been sufficiently oriented at that time.
The representative example of such a drawback of the conventional blow-molded bottle-shaped container exists in the fact that, when highly concentrated alcohol is contacted with the neck portion of such a bottle-shaped container, the contacting alcohol impregnates into the wall of the neck portion, resulting in white opacification of the neck portion due to crazing.
One method proposed for obviating this drawback in the conventional blow-molded bottle-shaped container of a biaxially oriented PET described above is to enhance the density of resin material by opacifying the material in white by utilizing the whitening phenomenon due to the crystallization of the resin material by heat treating the neck portion of the bottle-shaped container.
This method of strengthening the neck portion of the bottle-shaped container of the PET described above can greatly increase the craze resistance of the neck portion, improving the mechanical rigidity and producing other excellent effects such as impact resistance, wear resistance, and external pressure resistance of the bottle-shaped container. However, previously the neck portion of the bottle-shaped container thus strenghtened has always been thermally shrunk due to the influence of the heat treatment applied to whiten the resin material of the neck portion of the bottle-shaped container. Sometimes the neck portion is disadvantageously deformed due to the thermal shrinkage.
More particularly, the neck portion of the conventional bottle-shaped container of the PET described above of this type does not always have a uniform thickness at the peripheral wall due to the formation of a spiral thread or of a collar on the outer peripheral surface, nor been uniformly heated over the entire portion, nor been uniformly cooled simultaneously as a whole. Consequently shrinkage deformation of the neck portion does not always occur uniformly due to the thermal influence caused at the neck portion, sometimes causing the drawbacks and disadvantages of the loss of the circularity at the neck portion of the bottle-shaped container, and/or the loss of the flatness at the upper end face of the neck portion.